The present invention relates to optical filter architectures, and in particular to multiplexer and demultiplexer architectures for wave division multiplex optical signals.
Wave division multiplexed optical networks can utilise fused fibre coupler based wavelength multiplexers and demultiplexers. A fused fibre coupler 1 is shown in FIG. 1a, and as is known this has complementary sinusoidal frequency responses at its two output ports as shown in FIG. 1b. In the example shown in FIG. 1, channels or bands having wavelengths xcex1, xcex2, xcex3, and xcex4 entering the fused coupler input port X are split on exiting the fused coupler 1 between its output ports Y and Z. The frequency response corresponding to port Y is shown in solid line in FIG. 1b and it can be seen that attenuation is at a minimum at wavelengths xcex1 and xcex3, and at a maximum at wavelengths xcex2 and xcex4. Channels corresponding to wavelengths xcex1 and xcex3 are therefore passed out at port Y while channels corresponding to wavelengths xcex2 and xcex4 are not passed. The frequency response through port Z is shown in dashed line in FIG. 1b and is such that only channels corresponding to wavelengths xcex2 and xcex4 pass out of port Z. Fused fibre couplers and equivalent devices are used as sinusoidal filters. Fused fibre couplers are symmetrical as is known such that wavelengths fed into the Y port for example, can be split between the W and X ports in the same way that wavelengths through port X can be split between the Y and Z ports as described above.
FIG. 2 shows a typical fused coupler based wavelength demultiplexer structure 10 utilising a number of fused coupler elements or sinusoidal filters 11a-o arranged in a tree and branch structure or architecture. These arrangements rely on a series of channels having equally spaced wavelengths. In the example shown, a 16 channel demultiplexer comprises 15 fused coupler filter elements 11a-o arranged in a tree and branch structure. Each fused coupler filter element 11a-o is designed to pass half the channels entering its X port to its Y port, and the other half of the incoming channels to its Z port. The fused couplers filter elements 11a-o are arranged to have a pass band (ie minimum attenuation) corresponding to the lowest of the channel wavelengths entering their input ports (X). As discussed above in connection with FIG. 1, the filter elements 11a-o are arranged to split the incoming wavelengths or channels between their two output ports Y and Z. The sinusoidal frequency response of the filters 11a-o are arranged such that they may pass more than one of the wavelengths of the channel series. This is described in more detail with reference to FIG. 3 below which shows the demultiplexing stages of channel 1 by the filter series F, which comprises filter elements 11a, 11b, 11c and 11d. 
FIG. 3a shows the frequency response of fused coupler or filter element 11a. It can be seen that at the Y port, minimum attenuation is centred about wavelengths corresponding to channels 1, 3, 5, 7, 9, 11, 13, 15. These channels are then passed to the input port X of filter element 11b which has a sinusoidal frequency response shown in FIG. 3b and has a period double that of filter element 11a. It can be seen therefore that of the channels passed to input port X of filter 11b, channels 1, 5, 9, 13 are passed to output port Y of filter 11b. These channels are then fed to the input port X of filter element 11c, the frequency response of which is shown in FIG. 3c. Filter element 11c has a period double that of filter 11b, and hence quadruple that of filter 11a. It can be seen from FIG. 3c that only channels 1 and 9 will be passed to output port Y of filter element 11c. These are then fed to the input port X of filter element 11d which has a period 8 times that of filter element 11a, and its frequency response is shown in FIG. 3d. As can be seen, only channel 1 will be passed to output port Y of filter element 11d. It can be appreciated therefore that all channels 1-16 can be demultiplexed through various filter element series (e.g. F) as shown in FIGS. 2 and 3. Similarly, multiplexers can be constructed by a similar process as is known.
A major disadvantage with this type of demultiplexer is the high cost of fabrication. This is particularly significant in the deployment of wave division multiplex optical networks in the metro and access arenas.
It is an object of the present invention to provide an improved or at least alternative demultiplexer for use in wave division multiplexed optical networks.
In this specification the words multiplexer and demultiplexer are used interchangeably.
In a first aspect the present invention provides an apparatus or structure arranged to filter a predetermined range and number of wavelengths comprising:
a tree and branch filter architecture having a plurality of filters, each having a frequency response such that said filter passes a first set of wavelengths to a first output port and a second set of wavelengths to a second output port;
wherein at least two filters have the same frequency responses, and wherein each of said same frequency response filters are arranged to pass different wavelength sets.
Preferably said filters are sinusoidal filters.
Preferably said filters are fused fibre couplers.
Preferably said apparatus comprises a number
(n) of filter stages and wherein each stage has filters with a maximum of two different frequency responses.
Preferably said minimum attenuated wavelengths of the two frequency responses are:
(T1+T2+ . . . +Tn)/n and (Tn+1+Tn+2+ . . . +T2n)/n,
each having a period of 2(nxe2x88x921)T.
In a second aspect the present invention provides a demultiplexer comprising:
a plurality of devices each having at least one input port and two output ports, the frequency responses of the output ports of each device being complimentary in wavelength;
wherein the devices are connected in a tree and branch architecture to demultiplex a predetermined range and number of wavelengths;
and wherein at least two of the devices have the same frequency responses.
Preferably the devices are fused fibre couplers.
Preferably the demultiplexer comprises a number of stages and wherein each stage has devices with a maximum of two different frequency responses.
In a third aspect the present invention provides an apparatus arranged to multiplex or demultiplex a predetermined range and number of wavelengths comprising:
a plurality of devices each having at least one input port and two output ports, the output ports of each device having a periodic and complimentary series of minimum attenuation wavelength peaks;
wherein the devices are arranged such that each multiplexes or demultiplexer a unique series of wavelengths;
and wherein the apparatus comprises at least one device having a series of minimum attenuation wavelength peaks which do not correspond to the unique series of wavelengths the device is arranged to multiplex or demultiplex.
Preferably the devices are fused fibre couplers.
Preferably the demultiplexer comprises a number of stages and each stage has devices with a maximum of two different frequency responses.
Preferably each device in the last stage is replaced with a bi-directional module to enable said apparatus to simultaneously multiplexes and demultiplexes said wavelengths.
Preferably each said module comprises a four port device having the same frequency response as the device in the first stage, and two devices having the same two responses as the replaced devices.
In a fourth aspect the present invention provides an optical network or network segment comprising an apparatus or structure arranged to filter a predetermined range and number of wavelengths comprising:
a tree and branch filter architecture having a plurality of filters, each having a frequency response such that said filter passes a first set of wavelengths to a first output port and a second set of wavelengths to a second output port;
wherein at least two filters have the same frequency responses, and wherein each of said same frequency response filters are arranged to pass different wavelength sets.
In a fifth aspect the present invention provides an apparatus arranged to simultaneously multiplex and demultiplex a predetermined range and number of wavelengths comprising:
a plurality of devices each having at least one input port and two output ports, the output ports of each device having a periodic and complimentary series of minimum attenuation wavelength peaks;
wherein the devices are arranged such that each multiplexes and demultiplexes a unique series of wavelengths;
and wherein the apparatus comprises at least one device having a series of minimum attenuation wavelength peaks which do not correspond to the unique series of wavelengths the device is arranged to demultiplex.
Preferably the devices are arranged into a tree and branch architecture having a first device comprising two input ports, said ports having a periodic and complimentary series of minimum attenuation wavelength peaks being symmetrical with said output port peaks; the apparatus arranged such that a signal to be demultiplexed is fed to one said input port and a multiplexed signal is fed from the other said input port.
Preferably the devices are fused fibre couplers.
Preferably the demultiplexer comprises a number of stages and wherein each stage has devices with a maximum of two different frequency responses.